1. Field of the Invention
The present invention relates to a photo-sensor which converts a photo signal to an electric signal and amplifies the electric signal with a phototransistor, and to a measurement apparatus and a camera system using the same.
2. Description of the Related Art
A photo-sensor for photometry (hereinafter referred to as “AE sensor”) which is used in a digital single-lens reflex camera or the like needs to have a wide dynamic range performance that corresponds to a range from a low luminance to a high luminance and have a responsivity of following the change of incident light. As for the dynamic range, a photo-sensor is used which amplifies a photo-current with a phototransistor so as to enhance the measurement accuracy in a low luminance condition, during which a signal to be generated is small.
Such a photo-sensor has a problem that an electric current consumption increases in a high luminance period. For this reason, a method of limiting the electric current consumption of the photo-sensor to a fixed value is employed (hereinafter referred to as “current limiting”). For instance, a system of controlling a portion discharging excessive carrier, based on a signal quantity which has been output from a photoelectric conversion element is employed in a photoelectric conversion apparatus described in Japanese Patent Application Laid-Open No. 2000-244004.
On the other hand, as for the responsivity with respect to the incident light, for instance, a system of varying only an emitter potential without varying the base potential of the phototransistor is employed as is described in Japanese Patent Application Laid-Open No. 2000-77644. Furthermore, FIG. 5 in Japanese Patent Application Laid-Open No. 2000-77644 discloses a circuit configuration for performing the current limiting.
In the photoelectric conversion apparatus of FIG. 1 in Japanese Patent Application Laid-Open No. 2000-244004, when an electric current which has been amplified by the phototransistor exceeds a limit current, a gate potential of a MOSFET for discharging an electric charge abruptly increases due to a MOSFET for detecting an electric current. Then, the MOSFET for discharging an electric charge is turned ON, and thereby an excessive photo-current is discharged through the base of the phototransistor.
In other words, such a feedback circuit is formed that when a base current of the phototransistor changes, a drain voltage of the MOSFET for detecting an electric current changes, and a drain current of the MOSFET for discharging an electric charge changes. Then the base current of the phototransistor changes and the base potential of the phototransistor changes.
In this configuration, the MOSFET for discharging an electric charge controls the base of the phototransistor through a configuration of grounding the source, so once the MOSFET has been turned ON, the MOSFET continues to excessively discharge the photo-current until the gate potential changes by the feedback. A similar feedback occurs when the MOSFET for discharging an electric charge is turned OFF. The decrease of the stability due to a phase delay of this feedback can cause an oscillation state of making the MOSFET for discharging an electric charge switch repeatedly between ON and OFF.
When the MOSFET for discharging an electric charge serves as a reset switching MOSFET for an operation of resetting a pixel, which will be described later, the gate potential of the MOSFET cannot be directly controlled, so a complicated control becomes necessary. Furthermore, there has been a problem that the pixel cannot be reset in some operation conditions.
Also in a circuit which is shown in FIG. 5 of Japanese Patent Application Laid-Open No. 2000-77644, when a base current of the phototransistor changes, a drain voltage of the MOSFET for detecting an electric current changes. Then a drain current of the MOSFET for discharging an electric charge changes, and the base current of the phototransistor changes and the base potential of the phototransistor changes. Accordingly, the circuit of FIG. 5 has the same problem as that of the circuit shown in FIG. 1 of Japanese Patent Application Laid-Open No. 2000-244004.